1. Field of the Invention
The present invention relates to a magneto-optic disc for recording image, sound or code information.
2. Description of the Prior Art
Recently, a magneto-optic disk has attracted much attention as a high packing density recording medium. For example, in a conventional magneto-optic disk, a substrate has concentric circular pre-grooves, and a first protective film, a magnetic recording film, a second protective film, a reflection film and a protective coating are formed successively on the substrate. The substrate is made of an optically transparent material such as polycarbonate. The recording film is made of an alloy of a rare earth with transition metals such as TbFeCo. The protective films are made of, for example, silicon nitride and prevent the oxidation of the recording film. The reflection film is made of aluminum or an aluminum alloy and improves the detection efficiency of readout signals and controls the thermal diffusion. The protective coating is made of an ultra-violet-ray setting resin or the like and enhances the moisture-proof effect.
Magnetic field modulation recording is one of the methods of recording information in a magneto-optic disk. One of the advantages of this method is that it can perform direct overwrite of information and that it is appropriate for pulse length modulation recording.
In magnetic field modulation recording, while a laser beam not modulated and converged by an objective lens scans along the pre-grooves, the bias magnetic field is modulated simultaneously to change the direction of the magnetization at a portion of the magnetic recording film illuminated by the laser beam. The modulated bias magnetic field necessary for reversing the magnetization is generated, for example, with a floating magnetic head used in a magnetic disk drive (refer, for example, to Japanese Patent laid open Publication No. 229643/1988).
The modulated bias magnetic field is generated, for example, by supplying a modulated current to a coil of the magnetic head. The magnetic field is high enough to reverse the magnetization only in a restricted region near the magnetic head. Then, the distance between the illuminated portion of the magnetic recording film and the magnetic head has to be as short as possible.
The floating magnetic head comprises a magnetic head, a slider and a support mechanism. The magnetic head floats low on the protective coating due to the gas lubrication effect when the magneto-optic disk is rotated. Thus, the illuminated portion of the magneto-optic disk can be held within an effective range of magnetic field near the magnetic head. The contact start/stop (CSS) method makes it possible to use a compact structure of the magnetic head and the support of the slider. In the contact start/stop method, the magnetic head rests on the magneto-optic disk when the disk is not rotated, while it floats due to the gas lubrication effect when the disk is rotated.
In order to secure stable recording in the contact start/stop method, the magnetic head has to float smoothly on the magneto-optic disk. Therefore, the protective coating of the magneto-optic disk on which the slider slides has to satisfy the following conditions:
a) Foreign materials which may hinder the stable floating and sliding of the slider do not exist at the surface of the protective coating. PA1 b) The recording information of the magnetic recording film is not destroyed during contact of the disk with the magnetic head or the slider. PA1 c) The sliding and the floating of the slider is not hindered in any contact thereof with the protective coating. PA1 d) The absorption force between the protective coating and the slider which exceeds the motive force is not generated when the rotation of the magneto-optic disk is stopped.
In order to satisfy the above-mentioned conditions, it is suggested that a protective coating made from a thermosetting resin, alumina minute particles, a thermoplastic resin and a lubricant is applied to a recording medium and that a film of fluorinated carbon oil is further applied to the protective coating (Japanese Patent laid open Publication No. 229,643/1988). However, the inventors found that the floating stability of the magnetic head for a magneto-optic disk with such a protective coating is so bad as to cause many imperfect recording spots. By investigating the problem, the inventors found that alumina minute particles aggregate in the protective coating to form protrusions higher than the floating height of the slider at the surface of the protective coating. Thus, when a magneto-optic disk is rotated, such protrusions may collide with the slider or may obstruct the stable floating of the slider. Further, composites made from alumina minute particles and the thermosetting resin may be exfoliated from protrusions, so that they may scatter on the protective coating or adhere to the slider. Then, the stable floating of the slider is hindered, and the bias magnetic field applied to the magnetic recording film may become insufficient and increase errors.